Conventional hopper cars and trailers for transporting granular and comminuted materials such as grains, cement, flour and carbon black, are provided with bottom discharge surfaces having angles of repose up to and in excess of 45.degree. in order to ensure complete unloading by gravity. However, such inclined surfaces create substantial waste space in the cars or trailers while raising their center of gravity and increasing the construction cost thereof.
Certain prior unloading systems have attempted to overcome these disadvantages by providing collapsible bags or liners which are inflated by blower systems to lift the residual amount of material and create discharge surfaces at or above the required angle of repose to complete gravity discharge. U.S. Pat. No. 2,931,523 discloses such an unloading system. Obviously, such a system requires the availability and added expense of air pressure at the unloading site.
Another prior unloading system utilizing inflatable bags is disclosed in U.S. Pat. No. 3,139,998. This system requires primary and secondary inflatable bags, the primary bags carrying a higher pressure than the larger secondary bags to lift the material carried by the secondary bags to the required angle of repose for gravity discharge. Such a system is even more expensive than that of U.S. Pat. No. 2,931,523, and both systems require inclined frame members on which to mount the inflatable bags.